Magic Trick Helps Scientists Study How Brain Perceives the Body

Neuroscientists are using the “mirror box” illusion, a classic trick used by magicians, to reveal how the brain processes multiple sensory inputs to perceive our bodies and the world around us.

During this trick, a participant places his left hand on the table in front of a small mirror’s reflective surface. He then places his right hand behind the mirror, about six inches away, where he can’t see it.

Then he is asked to tap the table’s surface with both hands while looking at his reflection. Within a minute, he’ll feel as though the hand he sees reflected in the mirror is his right hand — even though the hidden hand did not move.

This classic mirror box illusion has been used in a number of neuroscience studies, including with amputees as a possible treatment to help reduce phantom limb pain, where it is believed to help the brain re-map and adapt to a missing limb.

Now, a new version of the mirror box illusion, developed by University of Delaware (UD) brain scientist Dr. Jared Medina and doctoral student Yuqi Liu, is pulling back more of the curtain on how the brain processes multiple sensory inputs to perceive our bodies and the world around us.

In this new take on the illusion, study participants placed their hands in opposite postures (one hand palm-up, the other palm-down), creating a conflict between visual and proprioceptive feedback for the hand behind the mirror.

Proprioception is your “sixth sense,” the sense of where your body is in space, that comes from your muscles and joints. This sense allows you to touch your nose with confidence even with your eyes closed.

After synchronous opening and closing of the two hands, the study participants suddenly perceived that the hand behind the mirror rotated or completely flipped to match the hand reflection.

“All of a sudden during our experiments, you’d hear a little laugh of surprise when people experienced this neat sensation of feeling like their hand flipped, even though it did not move,” Medina said.

The illusion’s effectiveness was influenced by the perceived difficulty of moving the hidden hand to the hand position seen in the mirror. Less illusion occurred for more difficult rotations requiring more strain.

Such biomechanical data, Medina said, is coded in the body schema, a representation of your body position in space that takes into account feedback from your senses, as well as stored information from muscles and joints about what your body can and can’t do.

According to Medina, the brain does “optimal integration” of incoming sensory information and then sorts out what the most reliable sense is.

“Vision is really precise,” Medina said, “but proprioception is noisier. So if there is a conflict between these senses, and vision is telling you that your hand is right there, but proprioception says it isn’t, your brain is optimally calculating.”

“Vision, because it is more precise, typically rules. However, in our study, the brain also appears to be considering additional information — biomechanical constraints from the body schema — in resolving this conflict between the senses.”

The research team is now using functional magnetic resonance imaging (fMRI) in UD’s Center for Biomedical and Brain Imaging to further investigate how the brain calculates and integrates the large amount of input it receives from all the senses.

The fMRI can reveal which regions of the brain are at work when performing a task. Having a better understanding of these brain processes could help researchers develop more advanced treatments for patients with brain injuries such as strokes, chronic pain, and other disorders, as well as develop artificial limbs that feel like a part of the body.

“How do you embody an artificial limb? It has to respect the laws of the body you’ve learned all your life,” Medina said.

“It’s quite important to figure out how the brain solves the problem of multisensory information, and how that relates to embodiment and our sense of self. These cool tricks and illusions are a path to understanding how the mind works.”

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About Traci Pedersen

Traci Pedersen is a professional writer with over a decade of experience. Her work consists of writing for both print and online publishers in a variety of genres including science chapter books, college and career articles, and elementary school curriculum.